The proposed work is designed to gain more complete understanding of the electrophysiological changes induced by myocardial ischemia and the effects of antiarrhythmic drugs on these changes. The specific purposes of the experiments are: 1) to define the determinants of and interrelationships between excitability conduction and refractoriness and the effect of ischemia on these determinants, 2) to determine the magnitude and inhomogeneity of changes in myocardial interstitial K associated with coronary artery occlusion and their role in the simultaneously occurring electrophysiological changes, 3) to determine the effect of drugs and other interventions on 1 and 2. Microelectrode techniques will be used to study the electrical changes in guinea pig ventricular muscle fibers and ion specific sensors will be used to study the interstitial electrolyte and electrical changes of in vivo pig hearts.